For transmission systems for transmitting data signals superposed on acoustic signals, there are a technique in which a transmission signal is diffusively superposed on an acoustic signal as described in Patent Literature 1 and a technique in which data is transmitted by utilizing a certain frequency band of an acoustic signal as described in Patent Literature 2. Such techniques for superposing data signals onto acoustic signals are typically used as digital watermarking for copyright protection and unauthorized copy prevention, and also as information communication techniques for transmitting information from a speaker to a microphone via sound waves.    Patent Literature 1: International Publication WO02/45286 pamphlet    Patent Literature 2: Japanese Patent Application Laid-Open No. 2001-148670